1. Field of the Invention
The disclosures herein generally relate to a memory circuit, and particularly relate to a nonvolatile memory circuit which is capable of retaining stored data in the absence of a power supply voltage.
2. Description of the Related Art
Conventionally, a nonvolatile memory cell requires a special structure such as a floating gate or a special material such as a ferroelectric material or ferromagnetic material for the purpose of achieving nonvolatile data retention. There is a new type of nonvolatile semiconductor memory device called PermSRAM, which uses a MIS (metal-insulating film-semiconductor) transistor as a nonvolatile memory cell (i.e., the basic unit of data storage). The MIS transistor used as a nonvolatile memory cell in PermSRAM has the same structure as ordinary MIS transistors used for conventional transistor functions (e.g., switching function), and do not require a special structure or a special material as described above. The absence of such a special structure and special material offers an advantage in cost reduction. PermSRAM was initially disclosed in PCT/JP2003/016143, which was filed on Dec. 17, 2003, the entire contents of which are hereby incorporated by reference.
PermSRAM is configured such that the MIS transistor used as a nonvolatile memory cell experiences an irreversible hot-carrier effect on purpose for storage of one-bit data. Here, the irreversible hot-carrier effect refers to the injection of carriers into the insulating film (i.e., oxide film) and/or sidewalls, which causes a change in the transistor's threshold voltage. Whether the threshold voltage has been changed due to a hot-carrier effect represents one-bit data “0” or “1”. Such a change in the threshold voltage may be detected by sensing a difference in ON current between the nonvolatile-memory-cell MIS transistor and a reference MIS transistor by using a sensing circuit such as a latch circuit.
In PermSRAM, data supplied from an external source may be initially written to the latch circuit. A store operation is then performed to transfer the data from the latch circuit to the nonvolatile-memory-cell MIS transistor. Whether the nonvolatile-memory-cell MIS transistor experiences a hot-carrier effect in the store operation depends on whether the data stored in the latch circuit is 0 or 1. A recall operation is subsequently performed to read the data stored in the nonvolatile-memory-cell MIS transistor. If the nonvolatile-memory-cell MIS transistor has experienced a hot-carrier effect in the store operation, an ON current smaller than the ON current of the reference MIS transistor may be detected in the recall operation. If the nonvolatile-memory-cell MIS transistor has not experienced a hot-carrier effect in the store operation, an ON current larger than the reference ON current may be detected in the recall operation. The latch circuit may be used as a sense circuit to sense such difference in ON current, and latches data that is either “0” or “1” depending on whether the ON current of the nonvolatile-memory-cell MIS transistor is larger or smaller than the reference ON current.
Carriers trapped in the insulating film and/or sidewalls of a MIS transistor due to a hot carrier effect include both electrons and holes. In general, an electron having enough kinetic energy in a semiconductor material can knock a bound electron out of its bound state to create an electron-hole pair. This phenomenon is known as impact ionization. In a case of an NMOS transistor, hot electrons having high-kinetic energy flowing through a transistor channel from the source to the drain cause impact ionization to occur at the channel/drain junction to create electron-hole pairs. Such impact ionization also occurs in a PMOS transistor. That is, holes having high-kinetic energy causes impact ionization at the channel/drain junction of a PMOS transistor to create electron-hole pairs.
Carriers trapped in the sidewalls are generally more sustainable than carriers trapped in the insulating film. That is, carriers trapped in the sidewalls tend to stay trapped longer than carriers trapped in the insulating film. For the purpose of thus retaining nonvolatile data, it is preferable to have a large number of carriers trapped in the sidewalls rather than to have a large number of carriers trapped in the insulating film.
There is thus a need for PermSRAM that is configured to induce the trapping of carriers in the sidewalls.